In the past all (consumer) graphics card BIOSes were smaller than 64 KBytes. With the introduction of GDDR5 additional space was required for the memory training code. Since ATI could not fit that piece of code into the 64 KB available, they increased the BIOS size to 128 KB (1 MBit).

For an unknown reason ATI's flashing software Winflash/Atiflash does not correctly detect those extra 64 KB and forgets to save them when you dump your BIOS to a file. The same happens to GPU-Z (fixed in 0.2.5 and newer). So everybody who extracted their HD 4870 BIOS ended up with no GDDR5 microcode.

I know this will in handy for those looking to change the fan speed profile, you can change it to run faster at lower GPU temps, thereby keeping the card cooler and more OC friendly with stock cooling.